The Effect of Using Layered Specimens for Determination of the Compressive Strength of Glass-ionomer Cements H.M. ANSTICE, J.W. NICHOLSON, and J.F. McCABE1 Materials Technology Group, Laboratory of the Government Chemist, Queens Road, Teddington, Middlesex, TWi1 OLY, United Kingdom; and 'Dental Materials Science Unit, Department of Prosthodontics, The Dental School, Newcastle-upon-Tyne, NE2 4BW, United Kingdom

Compressive strength is widely used as the criterion of strength of glass-ionomer dental cements, despite the difficulties in interpretation of the findings. With the introduction of light-cured glassionomer cements, which can be used only in thin layers, the question arises of how test specimens should be prepared for the measurement of compressive strength. A suggested method has been to prepare test pieces bybuildingthem up in layers, an approach which is examined critically in the current paper. Two different conventional (acid-base) glass-ionomers were studied with the use of layered and unlayered specimens of dimensions 6 mm (height) x 4 mm (diameter) and 12 mm (height) x 6 mm (diameter). While smaller samples gave the same value of compressive strength as larger specimens, layered specimens gave significantly lowervalues of compressive strength for both sizes. In view of these findings, and since the layered specimens are tedious to prepare, we conclude that compressive strength is unsatisfactory as a criterion ofstrength for light-cured glass-ionomer cements.

sive-strength specimen cannot be prepared from a single mix of cement. Despite this, manufacturers of commercial light-cured glass-ionomer cements quote values for the compressive strength of their materials, though they do not specify with their results how

Accepted for publication June 8, 1992 We acknowledge financial support (to HMA and JWN) under the 'Materials Measurement Programme", a programme of underpinning research financed by the United Kingdom Department of Trade and Industry. Crown copyright

Materials and methods. Cements were prepared by spatulation of known amounts of glass powder into an aqueous solution of poly(acrylic acid), with mixing being completed in 60-75 s. Test specimens were made with use of

these values have been obtained. One method that has been used has been for preparation of specimens of 6-mm height by 4-mm diameter in transparent molds. The cement is then cured by irradiation from the side, i.e., the direction of the light beam is at right angles to the axis of the cylinder. During irradiation, the cylinder needs to be rotated through 360'so that the specimen is cured evenly. The problem with this system is that it is difficult for uniform curing to be ensured on all sides of the specimen. This is because different parts of the specimen are irradiated at different times after being mixed, and the acid-base reaction will have proceeded further in those parts of the cylinder that are irradiated later. Moreover, it is difficult to ensure that any part of the specimen undergoes irradiation for the time recommended for clinical use, i.e., 20-30 s. Since the cure J Dent Res 71(12):1871-1874, December, 1992 carried out by this method is so different from that used clinically, the final material tested may differ quite significantly from the Introduction. material prepared for clinical use. Hence the validity of this test method is in doubt. Glass-ionomer cements have become widely used in clinical denAn alternative approach to the preparation of specimens for tistry following their introduction in the early 1970's (Wilson and compressive strength testingwas described recently (Iguodala-Cole McLean, 1988). As a measure ofthe performance characteristics of et al., 1991). Their study was aimed at determining the effect of these materials, the compressive strength has generally been deter- changing the specimen size on the measured compressive strength mined. For example, compressive strength is the criterion of of a light-cured glass-ionomer cement. The cement which they strength specified in the ISO Specification (ISO, 1986) and also the studied was Vitrebond (3M Health Care, St. Paul, MN), a cement British Standard (BSI, 1981) for glass-ionomer dental cements. The recommended as a lining material, and, rather than using a transminimum acceptable compressive strength of these materials is as parent mold and curing one-piece specimens, they used a layering follows: (i) for a Type 1 luting cement, at least 65 MPa, and (ii) for technique. It is not unusual to build up a light-activated specimen a Type 2 restorative material, at least 125 MPa. incrementally, when the material is used in a way similar to that Laboratory testing of dental restorative materials is carried out used in clinical placements. However, for the light-activated dental on specimens which bear no resemblance to dental restorations in cements in question, this is not the case. These products are shape, size, or method offabrication. Compressive strength is such normally used in thin layers which can be light-cured without a test. In both Standards, the test specimens, prepared in stainless incremental build-up. steel molds, are cylinders of 12-mm height and 6-mm diameter. In the study reported by Iguodala-Cole et al., specimens were More recently, however, following the widespread use of encapsu- composed of layers, each less than 2 mm deep in the two standard lated glass ionomers, the new Standard size of 6-mm height by 4- sizes (12 mm high x 6 mm diam and 6 mm high x 4 mm diam) as well mm diameter has been adopted to allow test specimens to be as in a new, non-standard smaller size of 6 mm high x 3 mm prepared from the contents of a single capsule (ISO DIS 9917, diameter, and they found no significant difference between the Dental water-based cements). mean values of compressive strength obtained from the three In 1988, the first modified glass-ionomer cements were intro- specimen sizes, although there was an increase in the scatter for the duced in which the initial cure was brought about photochemically smaller-sized specimens. following exposure to light (Antonucci et al., 1988; Mitra, 1988). The problem with these findings is that there remains doubt Since light can penetrate only a small depth ofmaterial during the about the validity of test specimens prepared in layers for determiphotocuring stage for such light-cured cements, the recommended nation of compressive strength, and this point was not considered by maximum layer for use clinically is 2 mm. Consequently, testing of Iguodala-Cole et al.. The hypothesis under test in the present study these materials becomes a problem, because a standard-compres- was that samples of glass-ionomer cement produced by the build-up in layers have the same strength as those produced in one piece. Received for publication February 20, 1992

Downloaded from at JOHNS HOPKINS UNIVERSITY on June 9, 2015 For personal use only. No other uses without permission.



J Dent Res December 1992

ANSTICE et al.











24.2 %



Split mould

Cylinder in mould

place in

modified standard split molds (Fig. 1). In these molds, a stainless steel cylinder of either 10 mm (height) by 6 mm (diameter) or 4 mm (height) by 4 mm (diameter) was placed inside the mold, leaving a depression 2 mm deep, into which the cement was packed. After this 2-mm depth of cement was allowed to cure, a washer 2-mm high was placed under the mold, so that the hole in the washer coincided with the position of the specimen in the mold. The specimen was then pushed down into the 2-mm gap provided by the washer, leaving a further depression (2-mm depth) for packing. This process was repeated until a cylindrical specimen of standard size had been made from either 6 or 3 layers of cement, depending on whether 6 mm x 12 mm or 4 mm x 6 mm specimens were being prepared. Each layer of cement was prepared from a freshly mixed batch of material. Two different formulations of conventional glass-ionomer cement were used. The first was Opusfil (Davis, Schottlander & Davis Ltd, UK), a commercially available glass-ionomer filling cement (type II), which was mixed according to the manufacturer's directions. The second cement was an experimental glass ionomer, made with the use of glass G338 prepared at LGC together with a 40% aqueous solution ofcommerciallyavailable poly(acrylic acid) [Versicol E7, Allied Colloids, Bradford, W. Yorkshire, UK]. G338 is a fluoroaluminosilicate glass, whose pre-firing composition is given in Table 1. This cement was mixed in the powder-to-liquid ratio of 3:1. For each cement, five groups of specimens were made. Two control groups were made which were of standard size (6 mm x 12 mm and 4 mm x 6 mm, respectively) and non-layered. In addition, three sets of layered specimens were made, with use ofthe apparatus described earlier. Layered specimens of size 6 mm x 12 mm (6 layers), 6 mm x 8 mm (4 layers), and 4 mm x 6 mm (3 layers) were


__--Split mould


Fig. 1-Modified split mold for preparation of specimens in layers.

prepared. Once prepared, the specimens were stored in their molds for one h at 37°C, and then, after this first hour, the specimens were removed from the molds and stored in water at 370C for a further 23 h prior to being tested. This is the storage procedure specified in both the ISO and British Standards. Eighteen specimens were prepared in each group. A few specimens broke on removal from the mold and were discarded. However, there were never fewer than 16 tested for any one type of sample. The load at failure in compression was measured in a Universal testing machine (Instron 1185), with cross-head speed of 1 mm/min.


Specimen Dimensions (mm) 6x 12

No. of Layers 1

Mean Strength (MPa) 144(A)


6x 12



6x 8


4x 6

Data Set 1


Probability of Failure at 100 MPa 0.015

S.D. 14

Characteristic Strength 150

Weibull Modulus (MPa) 10.5

143 (B)



















4x 6 3 109(B) 17 118 (i) From 16 to 18 samples were tested for each group. (ii) "1" for the number of layers indicates that the specimen was prepared conventionally. (iii) Specimens in groups A and B have means which do not differ significantly, p > 0.05, by ANOVA. (iv) Specimen dimensions are given as diameter x height. Downloaded from at JOHNS HOPKINS UNIVERSITY on June 9, 2015 For personal use only. No other uses without permission.


Vol. 71 No. 12 1.0 -













cod aoA




2 0

>% 0.5 -

00 .0 0



Of~~~1 *




0 0





A 0 0OA 00


O 0

3f O *A0* A *









N 0I


(U .0 .0










Compressive Stress (MPa)





Compressive Stress (MPa)

Fig. 2-Probability of failure against compressive stress for Opusfil. Solid circle = data set 1; opencircle = data set2; solid square = data set3; open square = data set 4; and triangle = data set 5. Refer to Table 2 for data-set attributes.

Fig. 3-Probability of failure against compressive stress for the experimental material. Solid circle = data set 6; open circle = data set 7; solid square = data set 8; open square = data set 9; and triangle = data set 10. Refer to Table 3 for data-set attributes.

From the load at failure, the compressive strength ofthe specimen was obtained.

< 0.05) for both of the cements at each of the sizes. Conversely, neither the specimen size nor the number of layers within layered specimens changed the compressive strength significantly (p > 0.05). The plots shown in Figs. 2 and 3 confirm these findings. In both Figs., the data points for unlayered specimens lie very close together, while those ofthe layered specimens he at lower stress values. Tables 2 and 3 show that the Weibull Modulus values for layered specimens were lower than those for unlayered specimens of equivalent size, indicating that layered specimens were less reliable. Tables 2 and 3 show that the unlayered 6 x 12-mm specimens had probabilities offailure at 100 MPa as low as did the unlayered 4 x 6mm specimens. All of the layered specimens had much greater probabilities of failure at a stress of 100 MPa, indicating that they were clearly weaker than the unlayered specimens.

ReM-sults. The results for groups of specimens were analyzed for mean strength (±SD), characteristic strength, Weibull modulus, and probability of failure at 100 MPa. Since the value of the Weibull modulus for each set of data was greater than 3, it was concluded that compressive strength was distributed in a near-Gaussian manner and the mean strength values could be compared by one-way analysis of variance. Differences between data sets were tested with a Student-NewmanKeuls procedure. Results for the determination ofcompressive strength for Opusfil for specimens of the various sizes, both layered and unlayered, are shown in Table 2. Similar results for the experimental cement are shown in Table 3. Figs. 2 and 3 give plots of probability of failure against compressive strength for each material. Statistical analysis of the data showed that the reduction in mean compressive strength in layered specimens was significant (p

Specimen Data Dimensions Set (mm)

The fact that smaller specimens give a greater scatter of results has been noted previously, both by Iguodala-Cole et al. (1991) and in the

TABLE 3 COMPRESSIVE STRENGTH DATA FOR THE EXPERIMENTAL MATERIAL Weibull Modulus No. of Mean Strength Characteristic S.D. Layers (MPa) Strength (MPa) 1 11 10.0 104 (A) 109 72 (B) 80 3.3 6 20


6 x 12


6 x 12


6x 8



4x 6


4x 6




80(B) 105(A) 76(B)

Probability of Failure at 100 MPa

0.355 0.873













(i) From 16 to 18 samples were tested for each group. (ii) "1" for the number of layers indicates that the specimen was prepared conventionally. (iii) Specimens in groups A and B have means which do not differ significantly, p > 0.05, by ANOVA. are given as diameter x height. (iv) Specimen dimensions Downloaded from at JOHNS HOPKINS UNIVERSITY on June 9, 2015 For personal use only. No other uses without permission.


J Dent Res December 1992

ANSTICE et al.

introduction to the British Standard on glass ionomers (BSI, 1981). We have confirmed that smaller specimens produced a wider scatter of results and were less reliable than larger specimens when the specimens were fabricated as a single layer. When specimens were constructed in more than one layer, however, reliability appeared to be unrelated to specimen size. Our results clearly demonstrated that specimens built up in layers gave values of mean compressive strength significantly lower than those of specimens prepared in one piece. When such specimens were crushed, however, fracture appeared to take place in the same way as for unlayered specimens, that is, along approximately conical lines that reflect the cones of stress within the specimen. There was no sign of fracture along the planes of the layers making up the specimens. This observation makes it difficult to explain why such layered specimens should give the observed decrease in measured compressive strength. The amounts of cement mixed at any one time varied considerably, dependingon the size ofthe specimenbeingprepared. However, this does not seem likely to have introduced differences in the materials' properties, because the smaller one-piece specimens gave the same value of compressive strength as the larger ones, yet the smaller ones were only 75.4 mm3 in volume compared with 339.3 mm3 for the larger specimens. For the larger layered specimens, each layer occupied a volume of 56.6 mm3, while for the smaller layered specimens, each layer was 3.1 mm3 in volume. Since the differences in the amounts of material for the larger and smaller one-piece specimens did not alter the compressive strength ofthe final cements, it seems unlikely that preparation of the smaller amounts required for the individual layers should have done so. For the cements described in the present study, no difference was found in strength of samples built up from different numbers of layers. The only difference was between layered and unlayered specimens. For both Opusfil and the experimental cement, the 6layered and the 4-layered specimens gave the same results, despite the change in the ratio of length to diameter between these sets of specimens. This contrasts with previous findings for zinc-oxide/ eugenol cements (Wilson, 1976), for which it was found that the length:diameter ratio did have an influence on the measured compressive strength. On the other hand, it is in agreement with the results of Groffinan and Wilson (1981), who found that specimen shape did not affect results from 0.7:1 to 4:1 length:diameter ratio for dental silicate cements. It is also in agreement with Iguodala-Cole et al. (1991) for Vitrebond, although the length:diameter ratio that they used varied only between 1.5:1 and 2:1. For conventional glass-ionomer cements, the testing of compressive strength has recently been called into question (McCabe et al., 1990). In a study of tests carried out in three different centers, three different groupings of results were obtained. These findings led to doubt about the current practice of pass or fail being assessed on the basis of arithmetic means rather than according to the probability of failure. McCabe et al. went on to conclude that, despite its widespread use, the compressive strength test is not suitable for inclusion in standards for glass-ionomer cements, and that an alternative means of evaluating these materials is needed. In addition to the difficulties reported by McCabe et al. (1990), there are fundamental difficulties in interpretation ofthe results of compression tests on glass-ionomer cements. This is because, for perfectly brittle materials, fracture does not formally happen by compression, but instead occurs either by the pulling apart ofplanes of atoms, i.e., tensile failure, or by the slippage of planes of atoms past each other, i.e., shear failure (Gillam, 1969). Inspection of fractured

test specimens shows that these types offailure do occur as a result of compression. However, the relationship between the true mechanical properties implicated in such failure and the recorded compressive strength is not at all clear. Failure in compression is complex, because both the mode and plane of failure are variable. Failure can occur by plastic yielding, cone failure (secondary shear forces), or by axial splitting (secondary tensile forces). In principle, the mode of failure depends on the size and geometry of the




as on



nature of the

material tested andthe rate ofloading. That being so, in order to have some chance at comparison ofthe properties of different cements, it is important that standard-sized specimens be used under standard conditions of loading (i.e., cross-head speed). Such considerations lead to the conclusion that, at best, compressive strength testing is really appropriate only for materials under completely standard conditions, which for glass ionomers means either 4 mm x 6 mm or 6 mm x 12 mm specimens loaded at a rate of 1 mm/min. For light-cured glass-ionomer cements, there is the added problem that uniform specimens ofthese sizes cannot easily be prepared. The suggested approach(Iguodala-Coleetal., 1991) ofbuildingup the test pieces layer by layer is not satisfactory, since, as has been shown by the present study, layered specimens give lower values ofcompressive strength than unlayered ones. Added to this, the preparation of layered specimens is a tedious and time-consuming process. We therefore conclude that fabricating cement specimens in layers significantly lowers the mean compressive strength and increases the probability of failure at relatively low stress levels. This has implications for the design of tests for light-cured glass-ionomer cements.

Acknowledgment. We thank Dr. A.D. Wilson (Eastman Dental Hospital, London) for helpful discussions during the preparation ofthis paper. REFERENCES Antonucci JM, McKinney JE, Stansbury JW, inventors (1988). (USSH) US Department of Health and Human Services, assignee. Resin-modified glass-ionomer dental cement, US Patent 7,160,856. July 26. British Standards Institute (1981). Specification for dental glass-ionomer cements. BS 6039. Gillam E (1969). Materials under stress. London (UK): Newnes-Butterworth. Groffman DM, Wilson AD (1981). The effect of certain experimental variables on the compressive strength of a dental silicate cement. JDent 9:116-125. Iguodala-Cole BO,AboushYEY,Vowles RW, EldertonRJ(1991). Challenging the ISO and BSI specifications. Effect of specimen size upon compressive and tensile strength measurements. Clin Mater 7:333-334. International Standards Organisation (1986). Dental glass polyalkenoate cements. ISO 7489. McCabe JF, Watts DC, Wilson HJ, Worthington HV (1990). An investigation oftest-house variability in the mechanical testing ofdental materials and the statistical treatment of results. JDent 18:90-97. Mitra SB (1988). European patent application 88,312,127.9 (Filed December 21). Wilson AD (1976). Examination ofthe test for compressive strength applied to zinc oxide eugenol cements. JDentRes 55:142-147. Wilson AD, McLean JW (1988). Glass ionomer cement. Chicago (IL): Quintessence.

Downloaded from at JOHNS HOPKINS UNIVERSITY on June 9, 2015 For personal use only. No other uses without permission.

The effect of using layered specimens for determination of the compressive strength of glass-ionomer cements.

Compressive strength is widely used as the criterion of strength of glass-ionomer dental cements, despite the difficulties in interpretation of the fi...
453KB Sizes 0 Downloads 0 Views